Conventional articles of athletic footwear include two primary elements, an upper and a sole structure. The upper provides a covering for the foot that securely receives and positions the foot with respect to the sole structure. In addition, the upper may have a configuration that protects the foot and provides ventilation, thereby cooling the foot and removing perspiration. The sole structure is secured to a lower portion of the upper and is generally positioned between the foot and the ground. In addition to attenuating ground reaction forces and absorbing energy, the sole structure may provide traction and control foot motions, such as pronation. Accordingly, the upper and the sole structure operate cooperatively to provide a comfortable structure that is suited for a variety of ambulatory activities, such as walking and running. The general features and configuration of the upper and the sole structure are discussed in greater detail below.
The upper forms a void on the interior of the footwear for receiving the foot. The void has the general shape of the foot, and access to the void is provided at an ankle opening. Accordingly, the upper extends over the instep and toe areas of the foot, along the medial and lateral sides of the foot, and around the heel area of the foot. A lacing system is often incorporated into the upper to selectively change the size of the ankle opening and permit the wearer to modify certain dimensions of the upper, particularly girth, to accommodate feet with varying proportions. In addition, the upper may include a tongue that extends under the lacing system to enhance the comfort of the footwear (e.g., to modulate pressure applied to the foot by the laces), and the upper also may include a heel counter to limit or control movement of the heel.
Various materials may be utilized in manufacturing the upper. The upper of an article of athletic footwear, for example, may be formed from multiple material layers that may include, for example, an exterior layer, a middle layer, and an interior layer (that fully or partially overlap). The materials forming the exterior layer (or other layers) of the upper may be selected based upon the properties of wear-resistance, abrasion resistance, flexibility, stretchability, and air-permeability, for example. With regard to the exterior layer, the toe area and the heel area may be formed of leather, synthetic leather, or a rubber material to impart a relatively high degree of wear-resistance and abrasion resistance. Leather, synthetic leather, and rubber materials, however, may not exhibit the desired degree of flexibility and air-permeability. Accordingly, various other areas of the exterior layer of the upper may be formed from a synthetic textile. The exterior layer of the upper may be formed, therefore, from numerous material elements that each imparts different properties to specific areas of the upper.
A middle (or other) layer of the upper may be formed from a lightweight polymer foam material that improves overall comfort and protects the foot from objects that may contact the upper. Similarly, an interior layer of the upper may be formed of a moisture-wicking textile that removes perspiration from the area immediately surrounding the foot. In some articles of athletic footwear, the various layers may be joined with an adhesive, and stitching may be utilized to join elements within a single layer or to reinforce specific areas of the upper.
The sole structure generally incorporates multiple layers that are conventionally referred to as an insole, a midsole, and an outsole. The insole (which also may constitute a sock liner) is a thin member located within the upper and adjacent the plantar (lower) surface of the foot to enhance footwear comfort, e.g., to wick away moisture. The midsole, which is traditionally attached to the upper along the entire length of the upper, forms the middle layer of the sole structure and serves a variety of purposes that include controlling foot motions and attenuating impact forces. The outsole forms the ground-contacting element of footwear and is usually fashioned from a durable, wear-resistant material that includes texturing or other features to improve traction.
The primary element of a conventional midsole is a resilient, polymer foam material, such as polyurethane or ethylvinylacetate (“EVA”) that extends throughout the length of the footwear. The properties of the polymer foam material in the midsole are primarily dependent upon factors that include the dimensional configuration of the midsole and the specific characteristics of the material selected for the polymer foam, including the density of the polymer foam material. By varying these factors throughout the midsole, the relative stiffness, degree of ground reaction force attenuation, and energy absorption properties may be altered to meet the specific demands of the activity for which the footwear is intended to be used.
Conventional polymer foam materials are resiliently compressible, in part, due to the inclusion of a plurality of open or closed cells that define an inner volume substantially displaced by gas. The polymer foam materials of the midsole may also absorb energy when compressed during ambulatory activities. The compression of the foam is affected by hysteresis loss, and deflection of such systems is affected by the volume of the compressed mass of the midsole.
There are several types of materials used to prepare midsoles. These include ethylene vinyl acetate (EVA), materials containing EVA, for example Phylon and Phylite, polyurethane and materials containing polyurethane. Ethylene vinyl acetate (EVA) is soft, light, and flexible. It is the least expensive midsole material and is often used in entry-level shoes. Midsoles are cut and shaped from flat sheets of EVA foam. EVA will compress and become flat over time as the air trapped within the foam is squeezed out. Once EVA is compacted, it does not return to its original shape and no longer provides cushioning. EVA compresses faster than other midsole materials. Phylon is made of EVA foam pellets, slabs, or sheets that are compressed, heat expanded, and then cooled in a mold. Compression-molded Phylon midsoles can be sculpted into a variety of designs that can be identified by their fine wrinkles Phylon is very lightweight, low-profile, and responsive. Phylite is an injection-molded unit made of a combination of 60% Phylon and 40% rubber. Phylite is lighter than rubber, but heavier than Phylon and functions as both midsole and outsole. Polyurethane is a dense, durable, and stable midsole material. Typically, polyurethane is poured into a mold to create a firm midsole that provides maximum protection from impact. Polyurethane is identified by its smooth rubbery feel and tendency to turn yellow with age. Polyurethane is the heaviest midsole material, but it is also the most durable.
One type of midsole configuration is described in U.S. Pat. No. 7,941,938. This midsole has a first portion having a lower ground engaging surface, an upper surface, and a recess formed in the upper surface, the upper surface of the first portion in contact with the upper; and a second portion is seated in the recess in the first portion. The second portion is formed from a first foam material comprising a reaction product of about 10 to about 100 phr hydrogenated or non-hydrogenated acrylonitrile butadiene copolymer, 0 to about 40 phr modified hydrogenated acrylonitrile butadiene copolymer, and 0 to about 90 phr alpha olefin copolymer.